The present invention relates to a stent for placement within the lumen of a vessel and, more particularly, to an intraluminal stent having end configurations that improve compliance matching of relative flexibility between the stent and the vessel to minimize flow disturbance and stress concentration in the artery wall.
Intraluminal, vascular stents have been widely used as solid mechanical, structural supports to maintain an open lumen following balloon angioplasty. However, there is a great deal of evidence that suggests that the mechanical environment in arteries plays an important role in the development and progression of cardiovascular disease. The presence of a vascular stent within the lumen of a vessel alters fluid flow patterns through the vessel. Accordingly, the use of stents may significantly influence the incidence of cardiovascular disease following balloon angioplasty due to changes in the mechanical environment caused by the stent.
From a mechanical perspective, arteries are quite complex. The arterial wall is subjected to internal pressurization that also induces large, circumferentially oriented stresses. In addition, the flow of blood through the artery creates a frictional shearing stress in the direction of flow. All of these stresses depend heavily on the arterial geometry, and thus vary greatly throughout the cardiovascular system. The presence of arterial grafts or stents provokes changes in these mechanical factors that may be important in determining the success of such grafts or stents.
Data are now emerging that implicate a mismatch in vessel compliance (i.e., the ratio of a change in vessel cross-sectional area to a change in vessel pressure) between the vascular graft or stent and the host vessel as a culprit in neointimal hyperplasia. Compliance mismatch is a mismatch in mechanical properties that results when a relatively rigid stent is inserted within a more flexible vessel. Compliance mismatch is a particular problem at the ends of the stent where the radial flexibility of the vessel abruptly changes due to the rigidity of an expanded stent. One of the consequences of compliance mismatch in a vessel is local blood flow alteration. For example, the presence of the stent in the vessel produces complex flow patterns that include vortices, which may lead to platelet activation, and flow stagnation, which may be related to platelet adhesion through increased near-wall particle residence time. Accordingly, a substantial mismatch between the circumferential compliance of arterial grafts or stents and contiguous native vessels may be deleterious to vessel patency. Accordingly, it would be desirable to reduce the problems associated with compliance mismatch.
In accordance with the present invention, an expandable, intraluminal stent is provided for deployment in a body passage, such as a blood vessel, to inhibit vessel stenosis. The stent in accordance with the present invention has a gradual change in radial flexibility along the longitudinal axis of the stent, in order to minimize flow disturbance and stress concentration in the artery wall, while still providing a relatively high radial strength enabling the stent to counteract elastic recoil of the vascular wall. In this regard, the stent in accordance with the present invention has increased radial flexibility at the outer ends to better match the flexibility or resiliency of the adjacent vessel while maintaining increased rigidity along the central portion of the stent to provide sufficient radial support to the vessel to inhibit vessel collapse or occlusion. In addition, the stent is easy to deploy, may be made of metal so that it can be imaged during deployment, and demonstrates a high expansion ratio. Furthermore, because of its unique configuration, the stent does not foreshorten along the longitudinal direction following expansion.
In a specific embodiment, a tubular stent includes a plurality of end links disposed in a ring at each end of the stent. Each end link is generally arcuate or V-shaped having a pair of legs joined at a central fulcrum. The tubular stent also includes a plurality of rigid longitudinal struts. The longitudinal struts are oriented generally parallel to one another about a central axis of the stent. The legs of the end links are connected to the ends of the longitudinal struts. The legs of a single end link are connected to the ends of adjacent longitudinal struts at one end of the stent.
At least one deformable link is circumferentially oriented around a central portion of the stent. The deformable link includes a plurality of individual V-shaped cross-links connected circumferentially between adjacent longitudinal struts. The length of the legs of the V-shaped cross-links can be changed to alter the rigidity of the expanded stent. The deformable cross-links enable the stent to be expanded from a first smaller diameter into a second larger diameter while providing sufficient rigidity to inhibit collapse of the vessel. The cross-links may have shorter legs than the legs of the end links so that shorter moment arms are provided by the cross-links so that the cross-links become more rigid than the end links.
The rigid longitudinal struts prevent longitudinal shrinkage of the stent during stent expansion. Toward this end, the longitudinal struts extend generally from one end of the stent to the other end of the stent. However, for applications where longitudinal foreshortening is not a problem or when greater longitudinal flexibility or greater radial flexibility about a central portion of the stent is desired, the longitudinal struts may span only part of the overall length of the stent. In a specific embodiment, one longitudinal strut can extend from one end of the strut toward a central portion of strut along one side of the tubular stent while another longitudinal strut extends from the other end of the stent toward a central portion of the stent along the opposite side of the tubular stent.
The end links are positioned with the legs oriented in an outward longitudinal direction to form moment arms so that radial flexibility of the end portions of the stent is increased. The stent permits a gradual change in radial flexibility from an end portion of the stent to the central portion of the stent to provide improved compliance matching between the stent and the vessel. The length of the legs of the end links may be changed to alter the radial flexibility of the stent for better compliance matching with the vessel.